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Triazoles fight unseen Asian rust infections

May 26, 2006

The spotty presence of Asian soybean rust within the United States leaves growers wondering how they'll treat the disease if it becomes a threat this growing season.

“We encourage growers not to take a wait-and-see approach with soybean rust due to the quick progression of the disease and the major impact it can have on yields,” says Randy Huckaba, product technology specialist for Dow AgroSciences. “Fields can become infected by soybean rust, yet not show visible symptoms for up to 10 days or more. And once you see symptoms, you may already have been too late to prevent some yield loss.”

Huckaba encourages growers and retailers to closely monitor confirmed reports of ASR, infection severity, and wind and weather patterns. If substantial ASR infections levels are reported within a one- to two-state distance upwind and soybeans are in the R1 to R6 stage, strong consideration should be given to preventive applications, he says.

When determining what product to use for first applications against ASR, Huckaba suggests looking at the type of control a fungicide brings to the field. Since a field can be infected by ASR without showing symptoms, a fungicide that offers both preventive and curative control is the most effective product, he says.

“The triazole class of chemistry offers this dual type of activity by itself, whereas the strobilurins and nitriles are preventive-only products. Some manufacturers have pre-mixed the strobilurins with a reduced rate of triazole chemistry to bring some curative activity to the table in an effort to do what the triazoles do by themselves.

“The bottom line is that for the first spray against soybean rust, make sure you cover all your bases by getting both preventive and curative control.”

Huckaba adds that the triazole chemistries may provide up to 21 days of residual control against ASR, depending upon disease pressure. For broader spectrum control, this chemistry is tank-mix compatible with strobilurin fungicides to control late-season disease such as frogeye leafspot or Septoria brown spot when these diseases and ASR are present or threatening an area.

“At that point, you've got control of soybean rust and you're making sure that it can't re-establish,” Huckaba says. “All fungicides provide preventive activity, although the length of residual control they offer may vary.”

Huckaba says he recommends that soybean growers use their best fungicides for first applications against ASR. Without the presence of late-season disease, for example, he recommends using a triazole like Laredo fungicide first, then rotating to a strobilurin or a triazole/strobilurin pre-mix fungicide second. Rotating classes of chemistry is a good resistance management tool.

Laredo fungicide is available for use on soybeans for the control of Asian soybean rust through Section 18 quarantine exemptions. The exemptions are valid through 2006 for use of this product in Alabama, Arkansas, Colorado, Delaware, Florida, Georgia, Iowa, Illinois, Indiana, Kansas, Kentucky, Louisiana, Maryland, Michigan, Minnesota, Missouri, Mississippi, North Carolina, North Dakota, Nebraska, New Jersey, Ney York, Ohio, Oklahoma, Pennslyvania, South Carolina, South Dakota, Tennessee, Texas, Virginia, Wisconsin and West Virginia.

State limitations exist regarding the number of applications that can be made using approved Section 18 products. For a list of retailers who stock Laredo, call (800) 258-3033. Always read and follow label directions.